It is known to provide a flexible plastic container which is conformable to a meat product such as poultry, ham, beef or bologna contained therein and whose inside wall portions are meat adherent during cook-in of the product in the package as for pasteurization, for example. The meat adherency of the inside wall portions enhances yields of the cook-in package meat product by minimizing and preferably substantially preventing cook-out, also referred to as purge, of fluids from the meat product in the plastic container in which the meat product is cooked. Examples of such containers include a stuffable type container wherein meat is initially compacted against the walls of a somewhat resilient container member such as a tubular casing or the like, and a container which has heat shrinkable characteristics such that the container shrinks at cooking temperatures for bringing the container in its sealed configuration into a close conforming condition relative to the meat contained therein so that meat product configuration changes during the cook-in process are compensated for by the container shrink characteristics.
U.S. Pat. No. 4,411,919 to Thompson discloses cook-in of packaged meat using an energetic radiation treated, meat adherent inner container surface comprising a polymeric olefin. U.S. Pat. No. 4,606,922 to Schirmer teaches using an irradiation treated ionomer layer as the meat adherent inside surface of a meat cook-in container. U.S. Pat. No. 4,888,223 discloses a multi-layer plastic film provided with an innermost layer of polyolefin resin which has been treated by corona discharge to improve its meat adhesion.
Such known meat adhering cook-in packages, particularly in certain use circumstances, display several disadvantages. These include that when the meat package encases meat having a retained skin, such as turkey or other poultry meat, the meat adherent interior surface of the package film can be so aggressively adherent to the skin as to result in undesirable delamination and separation of the skin from the cooked meat at the time when the package is removed from the meat following cook-in. Thus, as explained in U.S. Pat. No. 5,139,804 to Hoffman, even though the problem of purge or cook-out of fluids has been overcome, the resulting separation of skin from meat is considered to be commercially disadvantageous.
Another problem with the known cook-in packaging art involving a plastic container is that it is difficult, and apparently in some cases impossible, to correlate the amount of irradiation or corona discharge treatment or even ozone treatment uniformly given to a plastic surface with the subsequent degree of meat or other high protein adherence thereto. In such cases, the degree or extent of meat adherence does not appear to be regulatable by the amount of uniform surface treatment which is preliminarily given to the meat contacting surface portions of the intended cook-in container to be used for meat processing (preferably without purge).
Hoffman also states in U.S. Pat. No. 5,139,804 that a further problem in cook-in packaging art is that uniform surface treatment of the intended interior surface portions of the cook-in container, which treatment is carried out for the purpose of improving the capacity of highly proteinaceous foods to adhere thereto, appears to detract from the ability of the treated surface to adhere by heat sealing to similarly treated surfaces of plastics having even an identical composition. Hoffman explains that this is important because filled cook-in plastic containers are typically sealed before being exposed to cooking temperatures by heat sealing adjacent inner wall surface portions together as at the mouth or lip region of the container. Since steam and/or liquid pressures within the sealed container can develop, for example, in the cooking of beef at about 200.degree. F., or in cooking ham or turkey breast at 165.degree. F. for several hours, for example, unless durable container seals are formed, they tend to open during cooking with undesirable effects.
Hoffman reports that where the interior surface portions are treated with corona discharge, gamma radiation, ozone, etc., slight alteration in treated surface composition results. The chemical nature of this alteration is not known, but it is believed to involve at least some oxidation (perhaps partially oxidized) surfaces brought together and heat sealed, the resulting seal tending to be weaker than the corresponding seal formed between untreated surfaces. This effect thus presents a problem in providing containers for cook-in of meat and other highly proteinaceous foods when the container inside walls are to be both adherent to such foods in order to achieve, for example, a purge free cooked-in product package, and heat sealed together in formation of the package prior to cook-in. This problem can be avoided by using metal clips to close the package ends rather than heat sealing the package. However, when both good adhesion of the plastic film of the package to highly proteinaceous food and heat sealability of the film to itself or similar material for closing the package prior to cook-in are required, in the past one common practice has been to use a special, higher cost plastic film having both characteristics as discussed hereinafter, or to use a lower cost material such as a polymeric olefin for the film whose surface energy (wettability) is increased by a corona discharge treatment of the film in air so as to compromise the contradictory increase in adhesion and the decrease in heat sealability of the treated film to an amount sufficient for both meat adherence and heat sealing.
As an example, it is known to employ a relatively low cost, polymeric olefin film, particularly linear low density polyethylene film, in a heat sealable cook-in package for poultry by corona discharge treating the film in air to increase its initial surface energy from about 29 dynes/cm to about 42 dynes/cm to improve the meat adhesion characteristic of the film while still retaining a heat sealing capability. However, because of this compromise the package is not totally satisfactory in that it has a limited adhesion which makes it unsuitable to prevent purge for some meat formulations including ham, for example. If the surface energy of the film is increased to greater than 44 dynes/cm by the treatment, the film becomes impossible to heat seal and there is a tendency for the material to stick or adhere to itself on a roll to prevent or block unwinding the film from a roll.
A proposed solution to these problems as presented in U.S. Pat. No. 5,139,804 to Hoffman is to selectively surface treat the plastic materials in a patterned manner with the corona discharge. The preferred pattern has relatively small repeating units of treated and non-treated areas. Respective contacting surfaces heat sealed together with heat seals are obtained which display overall improved bond strengths. The selective surface treatment variations (corresponding to the treatment patterns) are such that areas of non-treatment in one surface align with other areas of non-treatment in the adjacent contacting surface intermittently along the length of an elongated sealed region formed by heat sealing. Hoffman states that the configurations associated with a treatment pattern can be used to enhance the increased bond strengths achieved in the heat sealed regions. In a disclosed embodiment in the patent, an engraved steel applicator roll is employed to provide the desired pattern of corona discharge treatment on the plastic film.
More recently, Wilhoit et al. proposed a cooked food product package, see U.S. Pat. No. 5,328,705, wherein an EVA-containing film surface is both irradiated and subjected to corona discharge treatment to provide an improvement in purge reduction and food adherence even through there is no commensurate increase in wetting tension of the treated surface over a corona discharge treated, but nonirradiated film. Starch particles are preferably dispersed across the meat contacting surface which is both irradiated and subjected to corona treatment. However, this and the aforementioned other proposed solutions to the above-referred to problems have not been totally satisfactory. For example, as noted above, corona discharge treatment of the film surface lowers heat sealability in the case of a polymeric olefin such as linear low density polyethylene. Corona discharge treated plastic film also has a relatively short shelf-life in that it loses its good properties of meat adhesion within a period of approximately 90 days. When stored longer than this the film must be retreated by corona discharge before use in cook-in packaging.
As mentioned before, special plastic films have been developed for overcoming these problems. Surlyn, a trademark of E.I. du Pont de Nemours & Co. for a group of ionomer resins, is an example of such materials. These materials are generally more costly and not always totally satisfactory. Some Surlyn materials, for example, can lose their meat adhesion property over an extended period of time such as twenty months. The expense of treatment equipment such as electron beam accelerators for irradiating plastic film also poses a difficulty, along with safety concerns as to the high-levels of radiation doses employed in the use of such equipment.
Cold plasma treatment of plastic films is, per se, known. For example, U.S. Pat. No. 3,870,610 to Baird, et al., discloses examples of treating polymeric materials in air and helium cold plasmas to improve their wettability and ink adhesion in preparation for printing. Thompson, in his aforementioned U.S. Pat. No. 4,411,919 refers to the patent to Baird, et al. as being of general interest in disclosing one of various oxidative treatments including corona discharge, flame, plasma, and ultraviolet light treatment contemplated by Thompson for use in improving the meat adherence of plastic film for cook-in packages. However, the examples disclosed by Thompson are limited to the highly energetic radiation treatments of irradiation by electron beam from an electron beam accelerator and corona discharge treatment, both being conducted in the presence of air. Disadvantages and drawbacks associated with making cook-in packages using such treatments and films have been referred to above.
Cold plasma treatment, also referred to as low temperature, 50.degree.-120.degree. C., plasma treatment has been employed to increase the tensile strength of polymer film as disclosed in U.S. Pat. No. 4,536,271 to Collins. Collins states that several modes of modification of the film properties by the cold plasma treatment include etching, cross-linking and coating of the film surface, depending on the treatment gas employed. Numerous possible organic and inorganic treatment gases are referred to by Collins in this regard. Other cold plasma process variables identified in the patent include the concentration of the treatment gas, absolute pressure during treatment, film exposure time to the reactive medium, and electrical power, voltage and current employed to excite the treatment gas. The patent to Collins does not relate to cook-in packages and does not address the aforementioned problems encountered in providing an improved cook-in package having a surface which has both well controlled, uniform meat adhesion and good heat sealability.
U.S. Pat. No. 4,772,348 to Hirokawa et al. discloses a method of making a gas barrier laminate of a thermoplastic layer and a layer of saponified copolymer of ethylene and vinyl acetate coextruded or thermally bonded to one another. The laminate is stated to be useful for forming containers or as a packaging film or sheet for food products, beverages and pharmaceutical products. The surface of the thermoplastic layer to be laminated with the layer of saponified copolymer of ethylene and vinyl acetate is pretreated with a low temperature plasma in a gaseous atmosphere of air, argon, nitrogen, oxygen, hydrogen, helium or a mixture of those gases, to permit lamination of the two layers. The thermoplastic layer is polyethylene terephthalate in a disclosed embodiment in the patent but according to the patentees it can be a thermoplastic film or sheet of a member selected from the group consisting of polyesters, polyamides, polycarbonates, polyvinyl chloride, polyolefins, polystrene and polyacrylonitrile, or a laminated film or sheet having at least one layer of one of said members. Hirokawa et al. are not concerned with nor do they suggest providing a cook-in package having an inner surface for contacting a proteinaceous food product such as a meat product, the inner surface having both a well controlled, uniform adherence to the food product and a good heat seal strength when heat sealed to a like surface for sealing the package before cooking.
Thus, there is a need for an improved cook-in package and a method of making the same which avoid the aforementioned drawbacks and limitations of the known cook-in packages and methods of making the same. An object of the present invention is to provide an improved cook-in package and method of making the cook-in package which solve these problems. More particularly, an object of the present invention is to provide an improved cook-in package comprising a cook-in film having a surface preferably formed of a relatively low cost polymeric olefin containing plastic material, which is both heat sealable to the package with an acceptable bond strength for sealing the package, and is capable of adhering in a well controlled and uniform manner to a highly proteinaceous food product such as meat cooked therein, wherein the improved cook-in package has a relatively long shelf-life with respect to retaining these characteristics as compared with the shelf-life of conventional packages.
Another object of the present invention is to provide an improved cook-in package and a method of making the same wherein meat adhesion of the film of the package is well controlled and uniform to prevent delamination and separation of the skin from the cooked meat where the meat has a retained skin, and to prevent overly aggressive adhesion of the film to itself when wound on a roll or to the meat after cook-in while still preventing purge during cook-in of various food products including ham, this being accomplished in a relatively safe, quick and inexpensive manner, without requiring the use of special, relatively expensive film material for the package.